Method and apparatus for heating a workpiece

ABSTRACT

In a furnace in which a workpiece is supported on hollow supporting members, a fluid heat transfer medium is circulated through a closed system which includes the hollow supporting members. The heat transfer medium which has a boiling point of at least 300* F. is introduced into the supporting member at a temperature of at least 300* F. and cooled when leaving the supporting members to a minimum temperature of 300* F. A pump is provided for circulating the heat transfer medium and a heat exchanger is provided between the exit side of the supporting members and the entry side of the pump. The medium in the closed system is preferably maintained at super-atmospheric pressures.

United States Patent 1 1 3,586,299

72] In e .lustinfiFoldessy 3,212,763 10/1965 Foldessy 6. 263/613 5 w g'gggg FOREIGN PATENTS kg 3 3" Nmfzo 1969 889,708 2/l962 GreatBritain263/68 [45] Patented [73] Assignee June 22, 19,71 United States SteelCorporation [54] METHOD ANT) APPARATUS FOR HEATING A 1,551,945 9/1925Fitch Primary Examiner-Charles .l. Myhre Attorney-Martin J. CarrollABSTRACT: In a furnace in which a workpiece is supported on hollowsupporting members, a fluid heat transfer medium is circulated through aclosed system which includes the hollow supporting members. The heattransfer medium which has a boiling point of at least 300 F. isintroduced into the supporting member at a temperature of at least 300F. and cooled when leaving the supporting members to a minimumtemperature of 300 F. A pump is provided for circulating the heattransfer medium and a heat exchanger is provided between the exit sideof the supporting members and the entry side of the pump. The medium inthe closed system is preferably maintained at super-atmosphericpressures.

PATENTED JUN22 [9n SHEET 1 BF 3 //v VEN TOR. JUS rm E. FOL 0533 yAttorney PATENTED JUN22 IHYI SHEET 2 BF 3 FIG. 2.

INVENTOR. JUST/N E. FOLDESSY By, /w41 Attorney PATENTEUJUH22197I 299SHEET 3 OF 3 INVENTOR. JUST/N E. FOLDESSY Attorney METHOD AND APPARATUSFOR HEATING A WORKPIECE This invention relates to a method and apparatusfor heating a workpiece and more particularly to the heating of steelbillets, slabs, blooms or the like which pass successively through aheating chamber on hollow skid rails which extend substantially thelength of the heating chamber. Each entering billet pushes the previousbillets along the skids with hot combustion gases being introduced bothabove and below the skids. The billets are pushed from the end of theskids onto a refractory lined bottom of a heated soaking chamber at theexit end of the furnace. The most common practice is to water cool theskids which results in a lower temperature in those parts of the billetwhich rest on the skids. In some instances this results in imperfectionsin the rolled product. To reduce or eliminate these skid marks somefurnaces have a plurality of longitudinally extending tunnels within thesoaking chamber beneath the skid marks with hot combustion gases passingthrough the tunnel. When open top tunnels are used they are soon filledwith scale from the billet which require shutdown of the furnace forremoval. When refractory covers are provided for the tunnels, thebillets break or wear away the covers to such an extent that the tunnelsare also filled with scale. According to my previous US. Pat. No.3,212,763 dated Oct. 10, 1965, the skids are air cooled rather thanwater cooled which permits the surface of the skid that contacts thebillet to attain a higher temperature so as to minimize the temperaturedifferential. This provides for more uniform heating of the billets, butmore careful control is required to prevent overheating of the skids.Also, the system to be economical requires that the heated air from theskids pass to recuperators to heat the combustion air or directly to theburners as combustion air. This requires a complicated system.

It is therefore an object of my invention to provide a furnace in whichthe skids are cooled with a cooling liquid having a boiling pointsubstantially above that of water so as to permit higher and moreuniform skid temperatures.

Another object is to provide a method of operating a furnace utilizing aheat transfer medium having a boiling point of at least 300 in which thetemperature of the medium entering and leaving the hollow skids isaccurately controlled.

These and other objects will be more apparent after referring to thefollowing specification and attached drawings, in which:

FIG. 1 is a schematic sectional elevation of a reheating furnace of thegeneral type shown in my above-mentioned patent;

FIG. 2 is a view taken on the line II-II of FIG. ll;

FIG. 3 is a view taken on the line III-III of FIG. 1; and

FIG. 4 is a schematic view of the cooling system for the furnace skids.

Referring more particularly to the drawings, reference numeral 2indicates the heating chamber of a continuous reheating slab furnacehaving an entry door 4 and an exit door 6. Hollow skids 8 extend fromthe door 4 to a refractory hearth section 10. The skids 8 are shown asbeing made in three sections spaced along the length of the furnace withfour skids arranged transversely of the furnace. This may be varied asdesired since it has no relationship to the present invention. The skidsB are connected to a coolant main 12 by means of vertical pipes 14 andtransverse headers 16. The other end of the skids 8 are connected to areturn main 18 by means of ver tical pipe 20 and headers 22. The skids 8are supported by transverse tubes 24 having one end connected to main 12by means of vertical pipes 26 and the other end connected to return main18. The transverse pipes 24 are supported by pipes 28 bent into the formof an inverted U. As shown in FIG. 3 one end of each pipe 28 isconnected to main l2 and the other end to main 18.

Referring now to FIG. 4 the skids 8 and pipes 24 and 28 are shownschematically connecting the mains l2 and 18. A storage tank 30 providedwith a heater 32 contains the heat transfer medium which is a liquidhaving a boiling point of at least 300 F. and preferably higher. Asuitable, readily available medium is HITEC manufactured by DuPont. Thisis a eutectic mixture containing 53' percent potassium nitrate, 40percent sodium nitrate, and 7 percent sodium nitrate. Dowtherm A orDowtherm E manufactured by Dow Chemical Company are also suitable,particularly the former. Dowtherm A is a eutectic mixture containing26.5 percent diphenyl and 73.5 percent diphenyl oxide by weight.Dowtherm E has as its main ingredient O-Dichlorobenzene.

One side of a transfer pump 34 is connected to tank 30 through a conduit36 having a valve 38 therein. The other side of the pump 34 is connectedto the tank 30 by means of a bypass 40 having a valve 42 therein. Mediumfrom the tank 30 is pumped to a pressure heater 44 through a conduit 46having a valve 48 therein and through a conduit 50. Also connected tothe pump 34 and conduit 50 is a conduit 52 having a valve 54 therein.The pressure heater 44 is provided with a burner 56 to which fuel issupplied through a thermostatic control valve 58. The pressure heater 44is provided with a vent 60. A circulating pump 62 connected to theheater 44 is connected to main 12 through a shutoff valve 64. A valvecontrolled vent 66 is provided in the highest part of the coolantsystem. The return main I8 is connected to a heat exchanger 68, which inturn is connected to the pressure heater 44. Located in the line betweenthe return main I8 and the heat exchanger 68 is a shutoff valve 70 and aback pressure valve 72. A bypass valve 74 is connected in parallel withback pressure valve 72. Cooling water is delivered to the heat exchanger68 through a conduit 76 having a control valve 78 therein. The settingof the valve 78 is controlled by means of a thermostat 80 locatedbetween return main l8 and valve 70. A conduit 82 connects the returnmain 18 to the tank 30. Shutoff valve 84 and drain pump 86 are connectedin series with conduit 82.

When the furnace has not been in operation for some time the heattransfer medium will be maintained at a temperature in the storage tankat a temperature between 53 and W0 F. To start the system the medium isfirst heated in the tank to a temperature preferably in the neighborhoodof 453. The vents 60 and 66 are opened, the valves 42, 48, 64, 70 and 74opened, and the valves 38, 54 and 84 closed. The pump 34 then transfersmedium from the tank 30 to the pressure heater expansion chamber 44where it is heated to and controlled at 450 F. The pump 62 is started inoperation to circulate the medium through the system at the same rate asthe pump 34 pumps the medium from the tank 30. When the system is filledpump 34 is stopped so that no more medium is transferred into thesystem. The temperature of the medium in the heater 44 is increased toand maintained at about 500 F. which causes the medium to at leastpartially vaporize with the vapor driving all the air from the system.The vents 60 and 66 are then closed and the valve 74 also closedresulting in the medium being circulated at a preset back pressurebetween and lbs. per sq. inch. During the filling operation only aminimum water flow is passed through the heat exchanger 68. The furnaceis now ready for firing and as the furnace heats up the medium iscirculated at the above pressure and temperature. Eventually the furnacewill be hot enough to heat the medium passing through the furnace to atemperature above 500 F. When the temperature of the medium in thereturn main 18 is above 750 F. the valve 78 will be controlled by thethermostat 80 so as to maintain the temperature at 750 F.

When the furnace is shut down for a short time the bypass valve 74 isopened when the temperature of the medium decreases below 450 F. so asto decrease the circulating pressure in the system. The temperatureregulators and the pressure heater 44 and heat exchanger 68 will be setto maintain a circulating temperature of 450 F. during the furnaceshutdown. When it becomes necessary to transfer the medium from thefurnace to the storage tank 30 for longer shutdowns, bypass valve 74will be opened when the medium temperature is below 450 F. When thetemperature of the medium in the system decreases to about 250 F. thevent 66 is opened, the valves 64, 42 and 48 closed and valves 38 and 54opened. The pump 34 is then operated to transfer the medium from theheater 44 to the storage tank 30. After the heater 44 is emptied theshutoff valve 36 is closed and the low points of the system are drainedby closing the valve 38 and opening the valve 84 and operating the drainpump 86 to deliver the medium through bypass 40 to the tank 30. When itis desired to again start the system the procedure is as describedabove.

While one embodiment of my invention has been shown and described, itwill be apparent that other adaptations and modifications may be made.

lclaim:

1. The method of operating a furnace in which a workpiece is supportedon hollow supporting members, which method comprises circulating a fluidheat transfer medium through a closed system including said hollowsupporting members, said medium having a boiling point of at least 300F., introducing said medium into said supporting member at a temperatureof at least 300 F., and cooling said medium leaving said supportingmembers to minimum temperature of 300 F. before returning it to saidsupporting members.

2. The method of claim 1 in which said medium is circulated at apressure substantially above atmospheric pressure.

3. The method of claim 2 in which said medium is circulated at apressure between approximately l30 and 145 pounds per sq. inch and is ata temperature not in excess of approximately 750 leaving the saidsupporting members.

4. The method of claim 3 in which said medium is introduced into saidsupporting members at a temperature of approximately 500 F.

5. in a furnace for heating a workpiece supported on hollow supportingmembers, the improvement comprising a closed cooling system includingsaid supporting members, a pump for circulating a fluid heat transfermedium having a boiling point of at least 300 F. through said coolingsystem, and a heat exchanger between the exit side of said supportingmembers and the entry side of said pump.

6. The combination of claim 5 including a back pressure valve in saidclosed cooling system to maintain said medium at superatmosphericpressure.

7. The combination of claim 5 including a heater for said heat transfermedium in said closed cooling system, a storage tank for said medium,and a transfer pump for transferring medium from said tank to saidheater.

8. The combination of claim 7 including means for controlling flow of acoolant to said heat exchanger, and means operable by the temperature ofsaid medium leaving said supporting members for operating said flowcontrolling means.

9. The combination of claim 7 including a back pressure valve in saidclosed cooling system to maintain said medium at superatmosphericpressure, a bypass around said valve, and a control valve in saidbypass.

10. The combination of claim 9 including means for draining the mediumfrom said closed cooling system and returning it to said storage tank.

1. The method of operating a furnace in which a workpiece is supportedon hollow supporting members, which method comprises circulating a fluidheat transfer medium through a closed system including said hollowsupporting members, said medium having a boiling point of at least 300*F., introducing said medium into said supporting member at a temperatureof at least 300* F., and cooling said medium leaving said supportingmembers to minimum temperature of 300* F. before returning it to saidsupporting members.
 2. The method of claim 1 in which said medium iscirculated at a pressure substantially above atmospheric pressure. 3.The method of claim 2 in which said medium is circulated at a pressurebetween approximately 130 and 145 pounds per sq. inch and is at atemperature not in excess of approximately 750* leaving the saidsupporting members.
 4. The method of claim 3 in which said medium isintroduced into said supporting members at a temperature ofapproximately 500* F.
 5. In a furnace for heating a workpiece supportedon hollow supporting members, the improvement comprising a closedcooling system including said supporting members, a pump for circulatinga fluid heat transfer medium having a boiling point of at least 300* F.through said cooling system, and a heat exchanger between the exit sideof said supporting members and the entry side of said pump.
 6. Thecombination of claim 5 including a back pressure valve in said closedcooling system To maintain said medium at superatmospheric pressure. 7.The combination of claim 5 including a heater for said heat transfermedium in said closed cooling system, a storage tank for said medium,and a transfer pump for transferring medium from said tank to saidheater.
 8. The combination of claim 7 including means for controllingflow of a coolant to said heat exchanger, and means operable by thetemperature of said medium leaving said supporting members for operatingsaid flow controlling means.
 9. The combination of claim 7 including aback pressure valve in said closed cooling system to maintain saidmedium at superatmospheric pressure, a bypass around said valve, and acontrol valve in said bypass.
 10. The combination of claim 9 includingmeans for draining the medium from said closed cooling system andreturning it to said storage tank.